fix: ensure terraform-docs is at 0.21.0

mentioned in merge request ucam-minimal-gcp-monitoring!61

mentioned in merge request gke-cluster!136

mentioned in merge request gcp-function!115

mentioned in merge request gcp-alert-snooze!141

mentioned in merge request ucam-observe-remote-server!100

mentioned in merge request gcp-monitoring!69

mentioned in merge request ucam-gcp-secret!111

mentioned in merge request gcp-container-image-mirror!109

mentioned in merge request gcp-site-monitoring!142

mentioned in merge request gcp-alert-to-gitlab-issue!47

mentioned in merge request gcp-pubsub-to-ms-teams!136

mentioned in merge request uis-traffic-manager-mtls!39

mentioned in merge request gcp-cloud-run-app!167

mentioned in merge request ucam-faas-long!108

mentioned in merge request gcp-secret-manager!113

mentioned in merge request ucam-faas!155